The present invention relates to engine diagnostics, and more particularly to apparatus for monitoring events in the combination chamber of an internal combustion engine by means of microwave energy injected into the chamber. Yet more specifically, the invention relates to an adapter for coupling both microwave and spark energy into the combustion chamber of a gasoline engine through an engine spark plug.
It has heretofore been proposed to inject microwave energy into the combustion chamber of a gasoline engine, and to monitor the microwave signal for identifying and diagnosing engine timing events, etc. The U.S. patents to Merlo U.S. Pat. Nos. 3,589,177 and 3,703,825 disclose an adapter for coupling both microwave and spark energy into the combustion chamber of a gasoline engine through an engine spark plug. The Merlo adapter includes a waveguide and a coaxial transmission line extending transversely through the waveguide to the spark plug in a generally T-shaped configuration. Spark energy is transmitted to the spark plug by the center conductor of the coax transmission portion of the adapter which extends transversely through the waveguide and is surrounded by an insulator. Microwave energy transmitted through the waveguide is coupled to the center conductor through the dielectric, and thereby to the combustion chamber through the spark plug conduction path and spark electrodes.
Spark plugs which have come into conventional and widespread use in recent years include a section of high resistance material (believed to be refractory material) or the like electrically separating the spark plug input terminal from the spark electrodes so as to inhibit passage of rf noise, etc. in the reverse direction from the electrodes to the spark plug leads. A significant disadvantage of the Merlo spark plug adapter previously described lies in the fact that it can operate only at greately reduced efficiency with such conventional spark plugs due to the fact that the Merlo adapter relies upon transmission of the microwave energy in the so-called coaxial TEM transmission mode through a spark plug conduction path from the input terminal to the electrodes. An important object of the present invention is to provide an adapter for coupling microwave energy into an engine combustion chamber through a spark plug which does not rely upon the spark plug terminal and conduction path, and which thereby avoids problems associated with the section of high resistance material in conventional spark plugs previously described.